This invention relates to combined plants having a steam turbine and a gas turbine connected together by a single shaft, and, more particularly, to a combined plant which is capable of operating in safety by avoiding overheating of the steam turbine that might otherwise occur due to a windage loss possibly caused by no load operation of the plant, or when operation is accelerated at the time of startup.
In this type of single-shaft combined plant of the aforemented type the steam turbine and gas turbine can be simultaneously started and accelerated. Thus, this type of plant offers the advantage that, as compared with multiple-shaft type combined plants in which the steam turbine and gas turbine are supported by separate shafts, it is possible to shorten the time required for achieving startup because the steam turbine and gas turbine can be simultaneously accelerated.
However, in this type of single-shaft combined plant, feeding of air to the steam turbine is not obtainable until the gas turbine is first accelerated and its exhaust gases are led to a waste heat recovery boiler to generate steam by using the exhaust gases as a heat source.
Generally, in a single-shaft type combined plant, the gas turbine can be usually accelerated to its rated rotational speed in about ten minutes following plant startup but the waste heat recovery boiler is unable to generate steam of sufficiently high temperature and pressure to supply air to the steam turbine in this period of time. Particularly the amount of waste heat released from the gas turbine is substantially proportional to the gas turbine load, so that it takes a prolonged period of time for the steam generating condition of the waste heat recovery boiler to be established when, for example, a no load condition prevails at the time of startup. Since the gas turbine and the steam turbine are connected together by a single shaft in a single-shaft type combined plant, the steam turbine can also attain its rated rotational speed in about ten minutes following plant startup. Prior to startup, the steam turbine has its interior evacuated with a vacuum pump, for example, to maintain the condenser in vacua. However, at plant startup, the pressure in the condenser is raised to a level higher than that prevailing in steadystate condition (or near the atmospheric pressure). If the turbine rotor rotates at high speed, the rotor temperature rises due to a windage loss. Particularly in the low pressure final stage of the turbine or stages near it, the rise in temperature due to a windage loss is marked because the turbine has elongated rotor blades and a high peripheral velocity. Centrifugal stresses developing in the roots of the blades are higher in the final stage and stages near it than in an initial stage of the turbine, so that if the temperature in this part of the turbine shows a marked rise in temperature due to a windage loss the material would undesirably be greatly reduced in strength.
In the event that the temperature of the steam in the inlet of a steam turbine shows an inordinate rise the turbine can be tripped by means of a safety device. The provision of the safety device raises the problem that the turbine is liable to be tripped due to a rise in the temperature of the final stage of the steam turbine at plant startup, thereby rendering plant startup impossible to accomplish.